In recent years, a segment-type friction material is under development as a wet friction material in order to improve material yield with resultant cost reduction and to lessen drag torque with resultant high fuel efficiency in a vehicle. The segment-type friction material has a flat ring shaped metal core and segment pieces made of friction material substrates cut into a segment piece shape along a circumference of the flat ring shape of the metal core. A set of the segment pieces is arranged and joined with an adhesive on one surface of the metal core side by side along an entire circumference of the metal core with an interval to be an oil groove. Another set of the segment pieces made of friction material substrates cut into a segment piece shape is joined on a rear surface of the metal core with an adhesive, too. Such segment-type friction material is applicable to a friction material clutch device that has a single or plural friction plates for use in an automatic transmission, which may be hereafter referred to as “AT”, of automobiles or motorcycles or the like.
As an example, a wet hydraulic clutch is used for an automatic transmission of an automobile or the like. The wet hydraulic clutch has a plurality of segment-type friction materials and a plurality of separator plates laid alternately on one another. Both the plates are contacted with a hydraulic pressure to transmit a torque. Lubricating oil or automatic transmission fluid, which may be hereafter referred to as “ATF”, is supplied to between both the plates for the purpose of absorbing frictional heat generated when they are shifted from a disengaged state to an engaged state or preventing abrasion of the friction material and the like. “ATF” is a registered trademark of Idemitsu Kosan Co., Ltd.
However, a distance between the segment-type friction materials and the separator plates associated therewith is set small in order to increase response of the hydraulic clutch. In addition, a total area of an oil passage provided on the segment-type friction material is restricted in order to assure a sufficient torque transmission capacity when the hydraulic clutch is engaged. As a result, there was a problem that drag torque was generated depending on quantity of oil flowing through the oil passage.
In order to solve such a problem, a patent document 1 discloses a wet friction member comprising oil grooves formed between adjacent ones of segment pieces, wherein an interval of the oil grooves becomes narrower from an inner peripheral side thereof toward an outer peripheral side thereof. Thereby, ATF running from the inner peripheral side toward the outer peripheral side of the oil grooves is blocked at a point where the interval of the oil grooves is changed and a part of ATF overflows and runs over surfaces of the segment pieces. Therefore, cooling effect by ATF can be improved while making heat resistance better and a drag torque can be reduced.    PATENT DOCUMENT 1: Japanese Patent Laid-Open Publication No. 2001-295859